The tightening of standards for exhaust gas components for motor vehicles worldwide and the further tightening as a result of emissions regulations not only requires a considerable reduction in engine emissions but also makes an extremely efficient exhaust gas treatment imperative.
SCR-technology, in particular, is becoming increasingly important for reducing nitrogen oxide. In these methods, which are already established, nitrogen oxide (NOx) contained in the oxygen-rich exhaust gas is reduced by means of ammonia. Preferably, in this connection precursor substances are drawn upon, in particular aqueous urea solutions which may be converted to ammonia. A high-purity eutectic solution of 32.5% urea in deionized water is marketed, for example, under the brand name AdBlue. The urea solution is hydrolized by means of hydrolyzing catalysts or directly on the SCR-catalyst to form ammonia and carbon dioxide. To this end, the urea solution is injected from an auxiliary reservoir in the vehicle into the exhaust gas flow by means of special metering systems upstream of the hydrolysis catalyst or the SCR-catalyst. The nitrogen oxide is then converted in the SCR-catalyst into nitrogen and water vapor. In this connection, there is the problem of ensuring a metering of the reducing agent which is as accurate as possible.
If correct and reliable metering is not ensured, an efficient removal of nitrogen oxide from the exhaust gas may not be achieved. An overdosage of reducing agent, on the other hand, may result in an undesired emission of ammonia, for example, a so-called break-through of reducing agent.
With liquid reducing agents, such as conventional urea solutions, the metering may take place by means of an injector. The activation time and thus the opening time of the injector, therefore, have an effect, amongst others, on the quantity of reducing agent supplied to the exhaust gas after-treatment system.
The AdBlue, which is preferably used as a reducing agent solution, generally has a high degree of purity and quality in accordance with DIN standard 70070 and the German Standard ISO 22241-1:2006(E). AdBlue, however, is liable to gradual decomposition and is only able to be stored for a limited time. During storage, it is converted into ammonia and carbon dioxide and may be concentrated in open systems by evaporation of the water component. The aqueous urea solution may then no longer fulfill the requirements of the aforementioned standards. At a storage temperature of 25° C. maximum, for example, the quality requirements for AdBlue are intended to be ensured for at least 6 months after its manufacture. If the recommended storage temperature is, however, exceeded, said time period is shortened. Thus the quality of the reducing agent is also fundamentally important when a requirement is determined for efficient removal of nitrogen oxide in the SCR-exhaust gas after-treatment system. In previously known exhaust gas after-treatment methods, the deviation in the quality of the reducing agent which may occur is, however, not taken into account as a factor influencing the determination of the requirement.
In DE 101 00 420 A1, for example, a method for controlling an exhaust gas after-treatment system for an internal combustion engine is disclosed, in which a predeterminable amount of reducing agent is supplied depending on the state of the internal combustion engine or the exhaust gas after-treatment system. The quantity of reducing agent supplied is thus varied according to the nitrogen oxide conversion of the SCR-catalyst depending on the detection of an overdosage and/or underdosage of the reducing agent. The method is complicated and dependent on many parameters, both of the internal combustion engine and the entire exhaust gas after-treatment system. In this exhaust gas after-treatment system and method, no allowances at all are made for the possible occurrence of decomposition or decrease in concentration of the reducing agent solution.
In previously known SCR-exhaust gas after-treatment systems, when calculating a requirement for the reducing agent and ultimately the metering thereof into the exhaust gas, a uniform quality, in particular a uniform concentration, is assumed. If the quality of the reducing agent is altered by one or more influencing factors, this may lead to a significant loss of efficiency of the SCR-catalyst.